In recent years, there has been a demand for reducing size and thickness of a digital camera. There has also been a demand for reducing size, thickness, and cost of an image pickup optical system mounted on a digital camera. To meet these demands, an image-pickup optical system which has a reduced number of optical elements and is provided with an optical element for bending an optical path, such as a prism, has been put into practical use.
In addition, there has been a demand for a telephoto zoom lens which accommodates high pixel counts of a solid-state image pickup device such as a CCD by finely correcting an aberration, as well as providing for a long focal length while being sufficiently bright, providing f-number in the order of 5.0 or less, in order to increase the added value of a product.
For example, Japanese Patent Laid-Open Publication No. Sho 57-138612 discloses a telephoto zoom lens constituted of a first group of lenses having a positive refracting power, second group of lenses having a negative refracting power, third group of lenses having a positive refracting power, and fourth group of lenses having a positive refracting power, from the object side in that order.
However, the telephoto zoom lens disclosed in Japanese Patent Laid-Open Publication No. Sho 57-138612 has a linear optical axis and is not suitable for an image pickup device such as a thin digital camera demanded nowadays.
However, Japanese Patent Laid-Open Publication No. 2003-202500, Japanese Patent Laid-Open Publication No. Hei 8-248318, Japanese Patent Laid-Open Publication No. 2000-131610, and Japanese Patent Laid-Open Publication No. 2005-173191 disclose an image pickup lens system constituted so that a high imaging performance can be obtained while it is compact and thin by providing an optical-path bending member such as a prism for bending the optical path for a first group of lenses.
FIG. 4 of Japanese Patent Laid-Open Publication No. 2003-202500 discloses an image pickup lens system in which a concave meniscus lens whose convex face is oriented toward the object side and a Plano convex lens whose convex face is oriented toward the object side are arranged from the object side in that order to the object side of a right-angle prism. Furthermore, FIG. 5 of Japanese Patent Laid-Open Publication No. 2003-202500 discloses an image pickup lens system in which a biconcave lens, biconvex lens, and Plano convex lens whose convex face is oriented toward the object side are arranged in that order from the object side to the object side of a right-angle prism.
That is, Japanese Patent Laid-Open Publication No. 2003-202500 discloses an image pickup lens system in which a lens system having a positive refracting power is disposed to the object side of a right-angle prism. However, in the case of the image pickup lens system in which a lens system having a positive refracting power is disposed at the object side of the right-angle prism shown in FIGS. 4 and 5 of Japanese Patent Laid-Open Publication No. 2003-202500 because lenses disposed to the object side of the right-angle prism are separated from each other, the optical axis at the object side of the reflection face of the right-angle prism becomes long. Therefore, this may be an obstacle in reducing size and thickness of a camera.
Moreover, Japanese Patent Laid-Open Publication No. Hei 8-248318 and Japanese Patent Laid-Open Publication No. 2000-131610 disclose an image pickup lens system in which a lens having a negative refracting power is disposed to the object side of a prism for bending an optical path as a lens constituting a first group of lenses. Furthermore, Patent Document 5 discloses an image pickup lens system in which a first group of lenses is constituted of a prism having a negative refracting power and a lens having a positive refracting power from the object side in that order.
The f-number is represented by “f/D”, where f is the focal length of a lens and D is the entrance pupil diameter (effective diameter) of the lens. Therefore, in order to secure a certain degree of f-number at the telephoto side having a long focal length, it is necessary to secure a certain degree of large value for the entrance pupil diameter D.
Therefore, when a telephoto zoom lens having a long focal length while maintaining image brightness is arranged so that the object side of the reflection face of a prism has a negative refracting power, as shown in Japanese Patent Laid-Open Publication No. Hei 8-248318, Japanese Patent Laid-Open Publication No. 2000-131610, and Japanese Patent Laid-Open Publication No. 2005-173191, an incoming light flux diverges relative to an entrance pupil diameter. Therefore, it is necessary to use a relatively large prism or a group of lenses disposed at the image face side of the prism, which may be an obstacle in reducing size and thickness of a camera.